1. Field of the Invention
The present invention relates to electric glow discharges, and more particularly, to a means and method for preventing damage to the electrodes due to arcing across a relatively large-volume electrically-excited glow discharge, such as in a laser, for example.
2. Description of the Prior Art
In a large volume electric plasma or a glow discharge, instabilities will result in a transition from a diffuse glow discharge to a highly localized metal vapor arc. These arcs cause significant erosion and damage to the electrodes if allowed to persist. The usual method for protecting a load from an arc includes connecting a "crowbar" device in parallel with the load, which is done in this invention also. The crowbar is a gas tube or a spark gap, for example, which has a very low impedance when triggered by a conventional control circuit used to sense a rapidly increasing current when an arc forms in the glow discharge. The voltage across the load is thus caused to fall to a low level insufficient to sustain the arc, and the arc is extinguished.
Other conventional methods of initiating the crowbar trigger employ the sensing of the load current, the load voltage, the rate of change of the load current or of the load voltage. All of these methods, however, are highly susceptible to electrical noise and transients so that the threshold level for detecting an overload current, for instance, must typically be set to 1.5 or 2 times the normal current level to avoid false triggering of the crowbar because of the difficulty of discriminating between an arc and electrical transients in the system. This type of crowbar control is unsatisfactory in cases where the electrodes are easily damaged by arcs, where the presence of an arc must be detected as early as possible, before the load current has increased significantly.
Other prior art known is the U.S. Pat. No. 3,801,821, for a Large Field Flash Sensor, which discloses a plurality of optical sensors with lens means to cover a large field of view, for detecting the presence and direction of one or more small flashing targets. When a target is detected, a warning light or tone is produced. Nothing is disclosed or suggested concerning the optical detection and prevention of electric arcs in a normal glow discharge.